1. Technical Field
The present invention relates to a thermally heated fixing device for use in an image forming apparatus employing electrophotography, and to an image forming apparatus such as a printer, a facsimile machine, a copier, and the like, including such a fixing device.
2. Related Art
In image forming apparatuses such as copiers, printers, and facsimile machines, a toner image is formed on an image carrier based on image data, the thus-formed toner image is transferred on a recording medium such as a sheet of paper or OHP film, and the recording medium carrying the toner image thereon is treated by a fixing device that fixes the toner image onto the recording medium using heat and pressure.
Fixing devices employing heated rollers are configured using a fixing roller and a pressure roller opposed to the fixing roller. The fixing roller is heated either directly or indirectly by a heat source such as a halogen heater or a heat coil employing induction heating. The two rollers are pressed against each other to form an area of contact across that is herein referred to as a nip portion. The recording medium carrying the image thereon is passed through the nip portion, so that toner forming the toner image is fused and fixed onto the recording medium using heat and pressure. The fixing roller method is widely employed due to its safety and its adaptability to high-speed printers.
The fixing roller has a metal core having a high thermal capacity. As a result, it takes several minutes for the fixing roller to reach a target temperature suitable for fixing the toner image on the recording medium. Therefore, this type of fixing roller must maintain a certain temperature even during standby when image formation is not performed, consuming a large amount of energy in the process.
A more energy-efficient type of fixing device that is frequently used is one employing a belt or a film, in which an insulation roller is heated externally, and further, only the area of the recording medium on which an image is formed is selectively heated based on the image data.
For example, there are configurations in which a planar heating member that contacts a cylinder of thin, heat-resistant film and a pressure roller together sandwich the film and the recording medium and press them against each other to impart thermal energy to the recording medium. Because the film is as thin as approximately 100 μm, the actual warm-up time is only the length of time needed to raise the temperature of the planar heating member having a low thermal capacity. Accordingly, the warm-up time can be shortened, thereby reducing the amount of power needed to warm up. Energy is saved by reducing power supply to a blank area (where no image exists on the recording medium), by changing a control temperature of the heating member and the area to be heated based on the image formed on recording medium.
Other approaches measure the temperature of each of multiple heat generators of a thermal heater and supply heat as appropriate, taking into account ambient temperature, and further, heating only the toner portions of the recording medium.
Additionally, the fixing roller may be heated externally. If the roller is heated from the outside, the heat remaining on the surface of the fixing roller can be used for fusing toner. Thus, the warm-up time is shortened compared to the internal-heating method that heats the entire fixing roller, thereby reducing energy loss. Moreover, the fixing device may be configured to selectively heat the image area alone and includes a second set temperature that is lower than the fixing temperature.
However, fixing devices that selectively heat the image area alone have a plurality of heating elements in both a sheet conveyance direction and the direction perpendicular to the sheet conveyance direction. In this case, each heating element may include variations in the heating due to initial or cumulative changes derived from manufacturing errors that in turn may produce high electrical density area and low electrical density areas.
Further, variations in the temperature of the belt that contacts the unfixed or blank image degrade image quality and cause defective image formation.